This invention relates to a metallic material as well as a method for manufacturing it from a high-temperature phase having an ultra-fine microstructure of a metal, the metal including an alloy which exhibits a phase transformation of a low-temperature phase into a high-temperature phase and vice versa. This invention also relates to a method for achieving an ultra-fine grain structure in a high-temperature phase as well as in a low-temperature phase derived from the high-temperature phase.
The terms "high-temperature phase" and "low-temperature phase" are used to mean phases appearing at a temperature higher or lower, respectively, than a transformation temperature, and the term "metal" is used to include a variety of metals in which a low-temperature phase is transformed into a high-temperature phase, such as steel, Ti, Ti-base alloys, Zr, Zr-base alloys, Ni, and Ni-base alloys. In the case of steel, the high-temperature phase is austenite and the low-temperature phase is ferrite, or the high-temperature phase is .delta.-ferrite and the low-temperature phase is .gamma.-austenite and in the case of titanium the former is .beta.-phase and the latter is .alpha.-phase. For brevity, however, this invention will be described using steel and Ti-base alloys as examples, and the low-temperature phase is ferrite or .alpha.-phase and the high-temperature phase is austenite or .beta.-phase.
It is well known that refining the grain structure of a metal produces improvements in properties of the metal such as its low temperature toughness, ductility, yield strength, corrosion resistance, and superplasticity. Thus, many processes to prepare a fine metallic structure have been developed.
However, prior art methods for refining the grain structure of a metal can attain a grain size of no smaller than 20 .mu.m in diameter. An industrial manufacturing method to provide a grain structure having an average grain size of 10 .mu.m or smaller in diameter, and generally 15 .mu.m or smaller has not yet been developed.
One industrial method for grain refining is the controlled rolling method. This is a method for preparing a fine grain structure for a hot-rolled steel material by controlling the hot rolling conditions, such as by lowering the finishing temperature to as low a level as possible. However, it is extremely difficult to obtain austenitic grains of the high-temperature phase which are 15 .mu.m or smaller in diameter. Therefore, there is a limit to the grain size of a ferritic structure which is derived from the above-described austenitic grains, and it has been thought to be impossible from a practical viewpoint to obtain a uniform and ultra-fine ferritic grain structure comprising grains having an average diameter of 10 .mu.m or smaller, especially 5 .mu.m or smaller.
The so-called accelerating cooling method has been developed for refining the grain size in a ferritic steel. In this method, the cooling rate is controlled after the completion of controlled rolling so as to increase the number of nuclei for the growth of ferritic crystal grains to further refine the crystal grains. However, according to this method, refinement of an austenitic structure before transformation occurs only during controlled rolling, and is not influenced by the subsequent cooling rate. Thus, there is still a limit to the grain size of an austenitic microstructure before transformation, and it is impossible to obtain a uniform, ultra-fine grained austenitic structure. Since austenitic grains are rather large, the martensite derived therefrom does not have a fine-grained structure.
Japanese Patent Publication No. 42021/1987 discloses a method of manufacturing hot rolled steel articles which comprises hot working a low-carbon steel with a high degree of deformation at a temperature higher than the transformation temperature to form a fine-grained ferritic structure so that recrystallization of austenitic grains can be prevented, and carrying out accelerated cooling to form bainite or martensite as well as to effect refinement of the thus-formed bainite or martensite. According to this method, a quenched structure which comprises ferritic grains having an average grain size of about 5 .mu.m with the balance being bainite or martensite can be obtained. However, the resulting bainite or martensite has an average grain size of 20-30 .mu.m. This is rather large.
The Japanese journal "Iron and Steel" Vol. 74 (1988) No. 6, pp. 1052-1057 discloses a method of manufacturing an ultra-fine austenitic grain structure by cold working an austenitic stainless steel (Fe-13/18 wt % Cr-8/12 wt % Ni) at room temperature to effect a strain-induced transformation of austenite into martensite, and annealing the resulting martensite by heating it at a temperature within a stable austenitic region to carry out reverse transformation of martensite into austenite, resulting in an ultra-fine austenitic grain structure. According to this method, a hot rolled stainless steel is subjected to cold rolling or a sub-zero treatment at a temperature lower than room temperature, and then is heated to a temperature in an austenitic region. This process corresponds to a conventional solution heat treatment of an austenitic steel. Such an ultra-fine microstructure can be obtained only for an austenitic high Cr-, high Ni stainless steel having a reverse transformation temperature of 500.degree.-600.degree. C. Therefore, as a general rule, it is impossible to obtain an austenitic microstructure having a grain size of 15 .mu.m or smaller for a common steel by the above-described method.